Element, chemical

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A chemical element can be defined in one of two ways: experimentally or
theoretically. Experimentally, an element is any substance that cannot be
broken down into any simpler substance. Imagine that you are given a piece
of pure iron and asked to break it down using any device or method ever
invented by chemists. Nothing you can do will ever change the iron into
anything simpler. Iron, therefore, is an element.

The experimental definition of an element can be explained by using a
second definition: an element is a substance in which all atoms are of the
same kind. If there were a way to look at each of the individual atoms in
the bar of pure iron mentioned above, they would all be the
same—all atoms of iron. In contrast, a chemical compound, such as
iron oxide, always contains at least two different kinds of atoms, in this
case, atoms of iron and atoms of oxygen.

Words to Know

Atomic mass:
The mass of the protons, neutrons, and electrons that make up an atom.

Atomic number:
The number of protons in the nucleus of an element's atom.

Chemical symbol:
A letter or pair of letters that represents some given amount of an
element.

Compound, chemical:
A substance that consists of two or more chemical elements joined to
each other in a specific proportion.

Metal:
An element that loses electrons in chemical reactions with other
elements.

Metalloid:
An element that acts sometimes like a metal and sometimes like a
nonmetal.

Nonmetal:
An element that tends to gain electrons in chemical reactions with
other elements.

Periodic table:
A system of classifying the chemical elements according to their atomic
number.

Synthetic element:
An element that is made artificially in a laboratory but is generally
not found in nature.

Natural and synthetic elements

Ninety-two chemical elements occur naturally on Earth. The others have
been made synthetically or artificially in a laboratory. Synthetic
elements are usually produced in particle accelerators (devices used to
increase the velocity of subatomic particles such as electrons and
protons) or nuclear reactors (devices used to control the energy released
by nuclear reactions). The first synthetic element to be produced was
technetium, discovered in 1937 by Italian American physicist Emilio
Segrè (1905–1989) and his colleague C. Perrier. Except for
technetium and promethium, all synthetic elements have larger nuclei than
uranium.

Two Dozen Common and Important Chemical Elements

Percent of all atoms*

Element

Symbol

In the universe

In Earth's crust

In sea water

In the human body

Characteristics under ordinary room conditions

*If no number is entered, the element constitutes less than 0.1
percent.

Aluminum

Al

—

6.3

—

—

A lightweight, silvery metal

Calcium

Ca

—

2.1

—

.02

Common in minerals, seashells, and bones

Carbon

C

—

—

—

10.7

Basic in all living things

Chlorine

Cl

—

—

0.3

—

A toxic gas

Copper

Cu

—

—

—

—

The only red metal

Gold

Au

—

—

—

—

The only yellow metal

Helium

He

7.1

—

—

—

A very light gas

Hydrogen

H

92.8

2.9

66.2

60.6

The lightest of all elements; a gas

Iodine

I

—

—

—

—

A nonmetal; used as antiseptic

Iron

Fe

—

2.1

—

—

A magnetic metal; used in steel

Lead

Pb

—

—

—

—

A soft, heavy metal

Magnesium

Mg

—

2.0

—

—

A very light metal

Mercury

Hg

—

—

—

—

A liquid metal; one of the two liquid elements

Nickel

Ni

—

—

—

—

A noncorroding metal; used in coins

Nitrogen

N

—

—

—

2.4

A gas; the major component of air

Oxygen

O

—

60.1

33.1

25.7

A gas; the second major component of air

Phosphorus

P

—

—

—

0.1

A nonmetal; essential to plants

Potassium

K

—

1.1

—

—

A metal; essential to plants; commonly called
"potash"

Silicon

Si

—

20.8

—

—

A semiconductor; used in electronics

Silver

Ag

—

—

—

—

A very shiny, valuable metal

Sodium

Na

—

2.2

0.3

—

A soft metal; reacts readily with water, air

Sulfur

S

—

—

—

0.1

A yellow nonmetal; flammable

Titanium

Ti

—

0.3

—

—

A light, strong, noncorroding metal used in space vehicles

Uranium

U

—

—

—

—

A very heavy metal; fuel for nuclear power

At the beginning of the twenty-first century, there were 114 known
elements, ranging from hydrogen (H), whose atoms have only one electron,
to the as-yet unnamed element whose atoms contain 114 electrons. New
elements are difficult to produce. Only a few atoms can be made at a time,
and it usually takes years before scientists agree on who discovered what
and when.

Classifying elements

More than 100 years ago, chemists began searching for ways to organize the
chemical elements. At first, they tried listing them by the size (mass) of
their nucleus, their atomic mass. Later, they found that using the number
of protons in their atomic nuclei was a more effective technique. They
invented a property known as atomic number for this organization. The
atomic number of an element is defined as the number of protons in the
nucleus of an atom of that element. Hydrogen has an atomic number of 1,
for example, because the nuclei of hydrogen atoms each contain
one—and only one—proton. Similarly, oxygen has an atomic
number of 8 because the nuclei of all oxygen atoms contain 8 protons. The
accompanying table (periodic table of the elements) contains a list of the
known chemical elements arranged in order according to their atomic
number.

Notice that the chemical symbol for each element is also included in the
table. The chemical symbol of an element is a letter or pair of letters
that stands for some given amount of the element, for example, for one
atom of the element. Thus, the symbol Ca stands for one atom of calcium,
and the symbol W stands for one atom of tungsten. Chemical symbols,
therefore, are not really abbreviations.

Chemical elements can be fully identified, therefore, by any one of three
characteristics: their name, their chemical symbol, or their atomic
number. If you know any one of these identifiers, you immediately know the
other two. Saying "Na" to a chemist immediately tells that
person that you are referring to sodium, element #11. Similarly, if you
say "element 19," the chemist knows that you're
referring to potassium, known by the symbol K.

The system of classifying elements used by chemists today is called the
periodic table. The law on which the periodic table is based was first
discovered almost simultaneously by German chemist Julius Lothar Meyer
(1830–1895) and Russian chemist Dmitry Mendeleev (1834–1907)
in about 1870. The periodic table is one of the most powerful tools in
chemistry because it organizes the chemical elements in groups that have
similar physical and chemical properties.

Periodic table of the elements.
(Reproduced by permission of

The Gale Group

.)

Properties of the elements

One useful way of describing the chemical elements is according to their
metallic or nonmetallic character. Most metals are hard with bright, shiny
surfaces, often white or grey in color. Since important exceptions to this
rule exist, metals are more properly defined according to the way they
behave in chemical reactions. Metals, by this definition, are elements
that lose electrons to other elements. By comparison, nonmetals are
elements that gain electrons from other elements in chemical reactions.
(They may be gases, liquids, or solids but seldom look like a metal.) The
vast majority (93) of the elements are metals; the rest are nonmetals.

A Who's Who of the Elements

Element

Distinction

Comment

Astatine (At)

The rarest

Rarest of the naturally occurring elements

Boron (B)

The strongest

Highest stretch resistance

Californium (Cf)

The most expensive

Sold at one time for about $1 billion a gram

Carbon (C)

The hardest

As diamond, one of its three solid forms

Germanium (Ge)

The purest

Has been purified to 99.99999999 percent purity

Helium (He)

The lowest melting point

−271.72°C at a pressure of 26 times atmospheric
pressure

Hydrogen (H)

The lowest density

Density 0.0000899 g/cc at atmospheric pressure and 0°C

Lithium (Li)

The lowest–density metal

Density 0.534g/cc

Osmium (Os)

The highest density

Density 22.57 g/cc

Radon (Rn)

The highest–density gas

Density 0.00973 g/cc at atmospheric pressure and 0°C

Tungsten (W)

The highest melting point

3,420°C

Historical background

The concept of a chemical element goes back more than 2,000 years. Ancient
Greek philosophers conceived of the idea that some materials are more
fundamental, or basic, than others. They listed obviously important
materials such as earth, air, fire, and water as possibly being such
"elemental" materials. These speculations belonged in the
category of philosophy, however, rather than science. The Greeks had no
way of testing their ideas to confirm them.

In fact, a few elements were already known long before the speculations of
the Greek philosophers. No one at that time called these materials
elements or thought of them as being different from the materials we call
compounds today. Among the early elements used by humans were iron,
copper, silver, tin, and lead. We know that early civilizations knew about
and used these elements because of tools, weapons, and pieces of art that
remain from the early periods of human history.

Another group of elements was discovered by the alchemists, the
semimystical scholars who contributed to the early development of
chemistry. These elements include antimony, arsenic, bismuth, phosphorus,
and zinc.

Formation of the Elements

How were the chemical elements formed? Scientists believe the answer to
that question lies in the stars and in the processes by which stars are
formed. The universe is thought to have been created at some moment in
time 12 to 15 billion years ago. Prior to that moment, nothing other
than energy is thought to have existed. But something occurred to
transform that energy into an enormous explosion: the big bang. In the
seconds following the big bang, matter began to form.

According to the big bang theory, the simplest forms of matter to appear
were protons and electrons. Some of these protons and electrons combined
to form atoms of hydrogen. A hydrogen atom consists of one proton and
one electron; it is the simplest atom that can exist. Slowly, over long
periods of time, hydrogen atoms began to come together in regions of
space forming dense clouds. The hydrogen in these clouds was pulled
closer and closer together by gravitational forces. Eventually these
clouds of hydrogen were dense enough to form stars.

A star is simply a mass of matter that generates energy by nuclear
reactions. The most common of these reactions involves the combination
of four hydrogen atoms to make one helium atom. As soon as stars began
to form, then, helium became the second element found in the universe.

As stars grow older, they switch from hydrogen-to-helium nuclear
reactions to other nuclear reactions. In another such reaction, helium
atoms combine to form carbon atoms. Later carbon atoms combine to form
oxygen, neon, sodium, and magnesium. Still later, neon and oxygen
combine with each other to form magnesium. As these reactions continue,
more and more of the chemical elements are formed.

At some point, all stars die. The nuclear reactions on which they depend
for their energy come to an end. In some cases, a star's death is
dramatic. It may actually blow itself apart, like an atomic bomb. The
elements of which the star was made are then spread throughout the
universe. They remain in space until they are drawn into the core of
other stars or other astronomical bodies, such as our own Earth. If this
theory is correct, then the atoms of iron, silver, and oxygen you see
around you every day actually started out life in the middle of a star
billions of miles away.

The modern definition of an element was first provided by English chemist
Robert Boyle (1627–1691). Boyle defined elements as "certain
primitive and simple, or perfectly unmingled bodies; which not being made
of any other bodies, or of one another, are the ingredients of which all
those call'd perfectly mixed bodies are immediately compounded, and
into which they are ultimately resolved." For all practical
purposes, Boyle's definition of an element has remained the
standard working definition for a chemical element ever since.

By the year 1800, no more than about 25 true elements had been discovered.
During the next hundred years, however, that situation changed rapidly. By
the end of the century, 80 elements were known. The rapid pace of
discovery during the 1800s can be attributed to the development of
chemistry as a science, to the improved tools of analysis available to
chemists, and to the new predictive power provided by the periodic law of
1870.

During the twentieth century, the last remaining handful of naturally
occurring elements were discovered and the synthetic elements were first
manufactured.